1. Field of the Invention
The present invention relates to a printer that prints an image with a head having nozzles that enable creation of dots having different quantities of ink.
2. Description of the Related Art
Ink jet printers that create dots with a plurality of color inks ejected from a plurality of nozzles formed on a head to record an image have been proposed as an output apparatus of a computer. The ink jet printers are widely used to print images processed by the computer in a multi-color, multi-tone manner. In such printers, each pixel is generally expressed by two tones, that is, the dot-on state and the dot-off state. The image is accordingly printed after the halftone processing, which is the image processing to enable the tones of original image data to be expressed by dispersibility of dots.
Multi-value printers, which are ink jet printers that enable expression of two or more tones, have recently been proposed to enrich the tone expression. Such printers include a printer that enables expression of three or more different densities by changing the quantity of ink or the density of ink and a printer that enables multi-tone expression by creating a plurality of dots in an overlapping manner in each pixel. The halftone processing is still required in such printers, since the tone of the original image data is not sufficiently expressible in each pixel.
In the multi-value printer, it is required to determine the recording ratio of each type of dot according to the tone value of the original image data in the course of the halftone processing. The conventional technique sets the recording ratio of each type of dot to appropriately express a variation in tone value and ensure the favorable granularity of the resulting printed image. Especially from the viewpoint of the improved granularity, there is a tendency of creating a large number of dots having a less quantity of ink.
In the multi-value printer that changes the quantity of ink, however, when a large number of a specific type of dot, which has a substantially identical size with a recording pitch of dots, banding often appears because of the reason discussed below.
FIG. 23 shows a state of recording only the specific type of dots in a predetermined image area. The rectangle shown on the left side of FIG. 23 represents a head with five nozzles. The open circles shown on the right side represent the specific type of dots. The hatched square denotes one pixel. In order to enable the whole image area to be filled with dots, the size of the specific type of dot is set to be substantially identical with or more precisely only a little greater than each side of the pixel, that is, the recording pitch of dots. In the example of FIG. 23, dots are created at the most ideal positions in the respective pixels. In this case, the predetermined image area can be filled uniformly with the dots.
In the ink jet printer, the respective nozzles generally have different ink ejecting characteristics, which cause a deviation of the dot recording positions. FIG. 24 shows a state of recording the specific type of dots with a deviation of the dot recording positions. In the illustrated example, ink is ejected in oblique directions from the first nozzle and the second nozzle, so that the positions of the dots created by the first nozzle and the second nozzle are deviated from the expected positions. The deviation of the dot recording positions causes unevenness of density or banding in the resulting printed image as clearly shown in FIG. 24. In an extreme case, there is a dropout between adjoining rows of dots.
FIG. 25 shows a state of recording another type of dots, which has a greater area than that of the specific type of dot, with a deviation of the dot recording positions. The symbols in FIG. 25 have the same meanings as those explained in FIGS. 23 and 24. Since there are significant overlaps of dots in the example of FIG. 25, the dots are expressed by the solid line and the dotted line alternately, for the clarity of illustration. There is no practical difference between the dots by the solid line and the dots by the dotted line. As clearly understood from the comparison with the example of FIG. 23, the dots shown in FIG. 25 have the greater size than each side of the pixel or the recording pitch of dots. This increases the overlapped area of the adjoining dots and thereby makes the unevenness of density, which is due to the deviation of the dot recording positions, relatively inconspicuous in the example of FIG. 25, compared with the example of FIG. 24. In the case of the specific type of dot having the substantially identical size with the recording pitch, even a little deviation of the dot recording positions makes the banding significantly conspicuous. The multi-value printer has been developed to enrich the tone expression and enable the high quality printing. The occurrence of the banding, which results in lowering the picture quality, is thus not negligible.